In recent years, there are increasingly severe demands on silver halide photographic emulsions, which are directed to photographic performances at still higher levels, e.g., higher sensitivity, superior graininess, higher sharpness, lower fog density and more sufficiently high optical density. In most cases, these demands apparently considered different from each other can be settled by techniques of preparing low-fog and high-speed silver halide emulsions. It is not too much to say that development of such low-fog and high-speed silver halide emulsions is a greatest subject in the present industrial field.
A most suitable method for achieving a higher sensitivity is to decrease inefficiency in the process of latent image formation of silver halide crystals so that the quantum efficiency in latent image formation can be improved. As a conventional means for improving this quantum efficiency, chemically sensitizing nuclei composed of silver sulfide, gold sulfide or a mixture of these that function as sensitivity centers capable of capturing free electrons are imparted so as to be formed on the surfaces, or in the insides, of silver halide crystals. This is commonly known as sulfur sensitization or gold-sulfur sensitization.
However, an attempt to use conventional chemical sensitization to form sensitizing nuclei having a good electron capture efficiency is known to result in the formation of a large number of sensitizing nuclei, which causes free-elecrtron capture competition between sensitizing nuclei to bring about a decrease in efficiency of latent image formation, i.e., a decrease in sensitivity. In addition, the size and physicochemical properties of a plurality of chemically sensitizing nuclei formed on the surfaces or insides of silver halide grains have not necessarily been made uniform.
As a means of overcoming the disadvantages in the conventional methods, techniques have been proposed in which chemical sensitization is carried out in the presence of what is called a chemical sensitization controlling agent or chemical sensitization modifier, to control the process of formation of chemical sensitization specks. For example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 126526/1983, U.S. Pat. Nos. 2,131,038, 3,411,914 and 3,554,757, and G. F. Duffin, "Photographic Emulsion Chemistry", The Focal Press Co., 1966, pp.138-143 disclose such techniques.
Various methods aiming at improving latent image forming efficiency by controlling the position of chemically sensitizing nuclei formed on silver halide grains are also disclosed in Japanese Patent O.P.I. Publications No. 93447/1986, No. 40938/1989, No. 62631/1989, No. 745540/1989, No. 201651/1989, No. 272743/1989, No. 345/1990 and No. 298935/1990. Japanese Patent O.P.I. Publication No. 158425/1989 discloses that the sensitivity efficiency can be improved by controlling the nucleus number ratio of chemically sensitizing nuclei on (100) face to those on (111) face in grains having (100) face and (111) face.
As a result of our studies, however, it has been revealed that the level of the recent demand for higher sensitivity can not be well achieved by these measures for improvements. That is, in these techniques in which the position or number of chemically sensitizing nuclei is controlled, what is intended is achieved basically by limiting the reaction areas of chemical sensitizers on silver halide grains, so that the reaction must be made to take place in relatively narrow regions to bring about the disadvantage that the control of the reaction process of chemical sensitization is so difficult that an attempt to sufficiently increase sensitivity tends to result in an increase in fog. Moreover, these techniques may be accompanied with an increase in low intensity reciprocity law failure and can not well satisfy the performances required in light-sensitive materials for photographing.
Meanwhile, Japanese Patent O.P.I. Publication No. 19804/1991 discloses a silver halide emulsion comprising grains on each surface of which two kinds of chemically sensitizing nuclei containing chalcogenides with different properties are present together. The disclosure in this publication, however, shows that one sensitizing nuclei preferentially increase the internal sensitivity of a silver halide grain. This is therefore clearly different from the constitution of the chemically sensitizing nuclei according to the present invention.
This publication has no disclosure as to the size of such sensitizing nuclei and also has no detailed disclosure as to the means for achieving the intended features. In addition, for the chemically sensitizing nuclei containing a chalcogenide capable of preferentially increasing the internal sensitivity as disclosed in that publication, it is difficult to be made stably present in an emulsion having been subjected to gold-sulfur sensitization, causing a problem of deterioration of storage stability of light-sensitive materials.